ผมเขียนติด Error ตรงนี้มาเป็นชั่วโมงแล้วคับ visual studio ภาษา C# นี่คือตัวโค้ด
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.OleDb;

namespace Order
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private DataSet _dataset;
        private OleDbConnection _conn;
        private OleDbCommand _command;
        private OleDbDataAdapter _adapter;
        private void Form1_Load(object sender, EventArgs e)
        {
            string conStr = "Provider = Microsoft.Jet.OleDb.4.0;" + "Data Source =" + Application.StartupPath + @"\AppSharp.mdb";

            _conn = new OleDbConnection(conStr);
            _conn.Open(); // ส่วนที่เกิด Error ขึ้น

            string sql = "SELECT * FROM Customers";
            _command = new OleDbCommand (sql, _conn);
            OleDbDataReader reader = _command.ExecuteReader();

            while (reader.Read())
            {
                cmbCustomer.Items.Add(reader["CustomerName"]);
            }

            sql = "SELECT * FROM Products";
            _command = new OleDbCommand(sql, _conn);
            reader = _command.ExecuteReader();

            while (reader.Read())
            {
                cmbProduct.Items.Add(reader["ProductName"]);
            }

            dateTimePicker1.Value = DateTime.Today;

            cmbCustomer.SelectedIndex = 0;
            cmbProduct.SelectedIndex = 0;

            btnDelate.Enabled = false;

        }

        private void formOrder_FormClosing(object sender, FormClosingEventArgs e)
        {
            _conn.Close();
        }

        private void showOrders()
        {
            string sql = "SELECT * FROM Orders ";
                 sql = "WHERE CustomerName = @cust";


            _command = new OleDbCommand (sql, _conn);
            _command.Parameters.AddWithValue("cust", cmbCustomer.SelectedItem);

            _adapter = new OleDbDataAdapter(_command);
            _dataset = new DataSet();

            _adapter.Fill(_dataset, "ord");

            dataGridView1.DataSource = _dataset.Tables["ord"];
        }

        private void cmbCustomer_SelectedIndexChanged(object sender, EventArgs e)
        {
            showOrders();
        }
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